Image reconstruction apparatus, image reconstruction method, and radiation tomography system

Abstract

To provide a device and a method capable of reconstructing a high-quality tomographic image based on measurement data collected by a WGI device.SOLUTION: The image reconstruction device 30 repeatedly performs the processing of the first image update unit 31, the second image update unit 32, and the energy function setting unit 33. The energy function setting unit 33 sets an energy function representing a degree of difference between the first tomographic image and the second tomographic image. The first image update unit 31 updates the first tomographic image by using the first measurement data collected by the absorption detector ring 20 for the coincidence event of the pair of gamma rays based on the energy function according to the update formula of the MAP reconstruction method. The second image update unit 32 updates the second tomographic image by using the second measurement data collected by the scattered detector ring 10 and the absorption detector ring 20 for the single counting event of one of the pair of gamma rays based on the energy function according to the update formula of the MAP reconstruction method.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image reconstruction apparatus, an image reconstruction method, and a radiation tomography system. [Background technology]

[0002] Positron Emission Tomography (PET) devices and Single Photon Emission Computed Tomography (SPECT) devices are known as radiation tomography devices that detect gamma rays generated within a subject and collect measurement data for reconstructing a tomographic image of the subject.

[0003] A PET device has a detector ring with multiple radiation detectors (absorption detectors) arranged around a measurement space in which a subject administered a drug labeled with a positron radioisotope (RI source) is placed. When a positron emitted from the RI source inside the subject annihilates with a nearby electron, a pair of gamma rays with an energy of 511 keV traveling in opposite directions is generated. Measurement data is collected for coincidence events in which this pair of gamma rays is detected by any two radiation detectors, and a tomographic image of the subject can be reconstructed based on this collected measurement data.

[0004] However, in a PET device, if one of a pair of gamma rays generated by a positron is not detected by the radiation detector, or if one gamma ray undergoes Compton scattering and its energy decreases before it enters the radiation detector, even if the other gamma ray with an energy of 511 keV is detected by the radiation detector and measurement data is obtained, the measurement data will not be used to reconstruct a tomographic image.

[0005] Non-Patent Document 1 describes a Whole Gamma Imaging (WGI) device as a radiation tomography device capable of collecting measurement data more efficiently. In addition to the functions of a PET device, the WGI device also has a Compton imaging function utilizing the principle of a Compton camera. With the WGI device, even if one of a pair of gamma rays generated by a positron is not detected by a radiation detector, or if one gamma ray is Compton scattered and its energy is reduced before it enters the radiation detector, it can reconstruct a tomographic image by Compton imaging using the measurement results of the other gamma ray. Furthermore, when a subject is administered a drug labeled with a positron radioisotope that also emits a single gamma ray, the WGI device can reconstruct a tomographic image by Compton imaging using the measurement results of a single gamma ray emitted from the positron radioisotope.

[0006] Like a PET device, a WGI device can reconstruct a first tomographic image using first measurement data collected for a coincidence event in which a pair of gamma rays with energy 511 keV is detected. Furthermore, a WGI device can reconstruct a second tomographic image using second measurement data collected for a single counting event in which one of the pair of gamma rays is detected, using an imaging technique similar to that of a Compton camera. In this way, compared to a PET device, a WGI device can more effectively utilize gamma rays generated within the subject, and can use more measurement data to reconstruct a tomographic image. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Takuya Takuya, Hideaki Tajima, Eiji Yoshida, Fumihiko Nishikido, Takumi Nishina, and Yasuga Yamatani, "Research and Development of Whole Gamma Imaging," MEDICAL IMAGING TECHNOLOGY, Vol. 39, No. 5, pp. 199-205 (2021). [Non-patent document 2] Peter J. Green, "Bayesian Reconstructions From Emission Tomography Data Using a Modified EM Algorithm," IEEE TRANSACTIONS ON MEDICAL IMAGING, Vol.9, No.1, pp.84-93 (1990). Summary of the Invention [Problem to be solved by the invention]

[0008] In general, in PET and SPECT devices, high quality tomographic images reconstructed based on measurement data are desired. In WGI devices, high quality is also desired for the first tomographic image reconstructed based on the first measurement data collected for coincidence counting events, and high quality is also desired for the second tomographic image reconstructed based on the second measurement data collected for single counting events.

[0009] The present invention has been made to solve the above problems, and aims to provide an apparatus and method that can reconstruct high-quality tomographic images based on measurement data collected by a WGI device. [Means for solving the problem]

[0010] The image reconstruction device of the present invention is a device that reconstructs a tomographic image of a subject based on measurement data collected by a scatter detector ring including a plurality of scatter detectors arranged around a measurement space in which a subject administered a drug labeled with a positron-emitting isotope is placed, and an absorption detector ring including a plurality of absorption detectors arranged around the scatter detector ring.The image reconstruction device of the present invention includes a first image update unit that updates a first tomographic image of the subject, a second image update unit that updates a second tomographic image of the subject, and an energy function setting unit that sets an energy function that represents the degree of difference between the first tomographic image and the second tomographic image.

[0011] The first image update unit uses first measurement data collected about a coincidence event in which a pair of gamma rays generated by positrons emitted from a positron radioisotope in the subject is photoelectrically absorbed and detected in any two of the multiple absorption detectors, and updates the first tomographic image using an update formula of the MAP reconstruction method based on the first measurement data and an energy function.

[0012] In a first aspect of the image reconstruction device of the present invention, the second image update unit uses second measurement data collected for a single counting event detected when one of a pair of gamma rays generated by a positron emitted from a positron radioisotope in the subject undergoes Compton scattering in one of the plurality of scattering detectors and then is photoelectrically absorbed in one of the plurality of absorption detectors, and updates the second tomographic image using an update equation for the MAP reconstruction method based on the second measurement data and the energy function. The image reconstruction device of the present invention repeats each of the processes of the energy function setting unit, the first image update unit, and the second image update unit multiple times while updating the energy function set by the energy function setting unit.

[0013] In a second aspect of the image reconstruction device of the present invention, when a drug labeled with a positron-emitting isotope that also emits a single gamma ray is administered to a subject, the second image update unit uses second measurement data collected for a single counting event in which a single gamma ray emitted from the positron-emitting isotope in the subject is Compton scattered in any one of the plurality of scattering detectors and then photoelectrically absorbed in any one of the plurality of absorption detectors, and updates the second tomographic image using an update equation for the MAP reconstruction method based on the second measurement data and the energy function. The image reconstruction device of the present invention repeats each of the processes of the energy function setting unit, the first image update unit, and the second image update unit multiple times while updating the energy function set by the energy function setting unit.

[0014] In the image reconstruction device of the present invention, the energy function has a larger function value the greater the difference between the first and second tomographic images, and it is preferable that the change in function value per increment of the difference be larger in the range where the difference is from the first threshold to the second threshold than in the range where the difference is from 0 to the first threshold and the range where the difference is greater than the second threshold.

[0015] The radiation tomography system of the present invention comprises a scatter detector ring including a plurality of scatter detectors arranged around a measurement space in which a subject administered a drug labeled with a positron-emitting isotope is placed, an absorption detector ring including a plurality of absorption detectors arranged around the scatter detector ring, and the image reconstruction device of the present invention described above that reconstructs a tomographic image of the subject based on measurement data collected by the scatter detector ring and the absorption detector ring.

[0016] The image reconstruction method of the present invention is a method for reconstructing a tomographic image of a subject based on measurement data collected by a scatter detector ring including a plurality of scatter detectors arranged around a measurement space in which a subject administered a drug labeled with a positron-emitting isotope is placed, and an absorption detector ring including a plurality of absorption detectors arranged around the scatter detector ring. The image reconstruction method of the present invention includes a first image updating step of updating a first tomographic image of the subject, a second image updating step of updating a second tomographic image of the subject, and an energy function setting step of setting an energy function that represents the degree of difference between the first tomographic image and the second tomographic image.

[0017] The first image update step uses first measurement data collected about a coincidence event in which a pair of gamma rays generated by positrons emitted from a positron radioisotope in the subject is photoelectrically absorbed and detected in any two of the multiple absorption detectors, and updates the first tomographic image using an update formula of the MAP reconstruction method based on the first measurement data and an energy function.

[0018] In a first aspect of the image reconstruction method of the present invention, the second image updating step uses second measurement data collected for a single counting event detected when one of a pair of gamma rays generated by a positron emitted from a positron-emitting isotope in the subject undergoes Compton scattering in any one of the plurality of scattering detectors and is then photoelectrically absorbed in any one of the plurality of absorption detectors, and updates the second tomographic image using an update equation for the MAP reconstruction method based on the second measurement data and the energy function. The image reconstruction method of the present invention repeats each of the processes of the energy function setting step, the first image updating step, and the second image updating step multiple times while updating the energy function set in the energy function setting step.

[0019] In a second aspect of the image reconstruction method of the present invention, when a drug labeled with a positron-emitting isotope that also emits a single gamma ray is administered to a subject, the second image updating step uses second measurement data collected for a single counting event detected when a single gamma ray emitted from the positron-emitting isotope in the subject undergoes Compton scattering in any one of the multiple scattering detectors and is then photoelectrically absorbed in any one of the multiple absorption detectors, and updates the second tomographic image using an update equation for the MAP reconstruction method based on the second measurement data and the energy function. The image reconstruction method of the present invention repeats each of the processes of the energy function setting step, the first image updating step, and the second image updating step multiple times while updating the energy function set in the energy function setting step.

[0020] In the image reconstruction method of the present invention, the energy function has a larger function value the greater the difference between the first and second tomographic images, and it is preferable that the change in function value per increment of the difference be larger in the range where the difference is from the first threshold to the second threshold than in the range where the difference is from 0 to the first threshold and the range where the difference is greater than the second threshold.

[0021] The program of the present invention causes a computer to execute each step of the image reconstruction method of the present invention. The recording medium of the present invention is a computer-readable medium having this program recorded thereon. [Effects of the Invention]

[0022] According to the present invention, high-quality tomographic images can be reconstructed based on measurement data collected by a WGI device. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a diagram showing the configuration of a radiation tomography system 1. As shown in FIG. [Figure 2] FIG. 2 is a diagram showing the configuration of the scattering detector ring 10 and the absorption detector ring 20. [Figure 3] FIG. 3 is a diagram for explaining an event (coincidence event) in which two absorption detectors 21 simultaneously detect a pair of gamma rays with energy of 511 keV. [Figure 4] FIG. 4 is a diagram illustrating an event (single counting event) in which one scattering detector 11 and one absorption detector 21 detect a single gamma ray. [Figure 5] FIG. 5 is a flowchart of the image reconstruction method. [Figure 6] FIG. 6 is a diagram illustrating an example of an energy function. [Figure 7] FIG. 7 is a diagram showing another example of the energy function. [Figure 8] FIG. 8 is a diagram showing the configuration of the phantom 3 assumed in the simulation. [Figure 9] Fig. 9(a) is a diagram showing a first tomographic image reconstructed in the comparative example, and Fig. 9(b) is a diagram showing a second tomographic image reconstructed in the comparative example. [Figure 10] 10(a) is a diagram showing a first tomographic image reconstructed in the case of Example 1. FIG. 10(b) is a diagram showing a second tomographic image reconstructed in the case of Example 1. [Figure 11]11(a) is a diagram showing a first tomographic image reconstructed in the case of Example 2. FIG. 11(b) is a diagram showing a second tomographic image reconstructed in the case of Example 2. [Figure 12] FIG. 12 is a graph showing the relationship between the resolution and noise of the first tomographic image reconstructed in the simulation. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate explanations will be omitted. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0025] Fig. 1 is a diagram showing the configuration of a radiation tomography system 1. The radiation tomography system 1 includes a scatter detector ring 10, an absorption detector ring 20, and an image reconstruction device 30. Fig. 2 is a diagram showing the configuration of the scatter detector ring 10 and the absorption detector ring 20. The image reconstruction device 30 includes a first image update unit 31, a second image update unit 32, an energy function setting unit 33, and a storage unit 34. The image reconstruction device 30 can be configured using a computer, and preferably includes a GPU to perform each process at high speed.

[0026] The scatter detector ring 10 includes a plurality of scatter detectors 11 arranged around a measurement space in which a subject 2 administered with a drug labeled with a positron radioisotope (RI source) is placed. A positron emitted from the RI source in the subject 2 annihilates with a nearby electron, generating a pair of gamma rays with an energy of 511 keV traveling in opposite directions. When one of the pair of gamma rays is incident on the scatter detector 11, the gamma ray may pass through the scatter detector 11 without undergoing Compton scattering, or may undergo Compton scattering in the scatter detector 11. When the incident gamma ray undergoes Compton scattering, the scatter detector 11 detects the scattering event. The absorption detector ring 20 includes a plurality of absorption detectors 21 arranged around the scatter detector ring 10. The absorption detector 21 photoelectrically absorbs the incident gamma ray and detects the absorption event.

[0027] When any two of the multiple absorption detectors 21 included in the absorption detector ring 20 perform coincidence counting of a pair of gamma rays with energy 511 keV, that is, when the two absorption detectors 21 simultaneously detect an absorption event of gamma rays with energy 511 keV, first measurement data is collected for this coincidence event, representing a line (coincidence line) L connecting the positions P1 and P2 of the two absorption detectors 21. The RI source that emitted the positron exists on this coincidence line L (see FIG. 3).

[0028] If a scattering event detected by any one of the multiple scattering detectors 11 included in the scattering detector ring 10 and an absorption event detected by any one of the multiple absorption detectors 21 included in the absorption detector ring 20 occur simultaneously, it is determined that a single gamma ray has undergone Compton scattering in the scattering detector 11 and then been photoelectrically absorbed and detected in the absorption detector 21. Second measurement data is collected for the single counting event in which this single gamma ray was detected.

[0029] The second measurement data includes the position P1 where the gamma ray underwent Compton scattering in the scattering detector 11, the energy E1 lost by the gamma ray when the gamma ray underwent Compton scattering in the scattering detector 11, the position P2 where the gamma ray was absorbed in the absorption detector 21, and the energy E2 lost by the gamma ray when the gamma ray was absorbed in the absorption detector 21. From these data, the Compton scattering angle θ can be calculated. Furthermore, if a cone is imagined with the line connecting the scattering position P1 and the absorption position P2 as its central axis and the scattering position P1 as its vertex, with an angle θ between the central axis and its generatrix, then the RI radiation source that emitted the positron exists on the surface of this cone (see FIG. 4).

[0030] The storage unit 34 stores programs for executing the processing of the first image update unit 31, the processing of the second image update unit 32, and the processing of the energy function setting unit 33. The programs stored in the storage unit 34 may have been stored at the time of manufacturing or shipping the image reconstruction device 30, or may have been acquired via a communication line after shipping and then stored in the storage unit 34, or may have been recorded on a computer-readable recording medium 40 and then stored in the storage unit 34. The recording medium 40 may be any type of medium, such as a flexible disk, CD-ROM, DVD-ROM, BD-ROM, or USB memory.

[0031] The memory unit 34 also stores first measurement data collected for coincidence events and second measurement data collected for single counting events. The first measurement data is collected for coincidence events in which a pair of gamma rays generated by positrons emitted from the RI source in the subject 2 are photoelectrically absorbed and detected in any two absorption detectors 21. The second measurement data is collected for single counting events in which one of a pair of gamma rays generated by positrons emitted from the RI source in the subject 2 is Compton scattered in any one scattering detector 11 and then photoelectrically absorbed and detected in any one absorption detector 21.

[0032] The first image update unit 31 updates the first tomographic image of the subject 2 using the first measurement data collected for the coincidence counting events and stored in the memory unit 34. The second image update unit 32 updates the second tomographic image of the subject 2 using the second measurement data collected for the single counting event and stored in the memory unit 34. The energy function setting unit 33 sets an energy function that represents the degree of difference between the first tomographic image and the second tomographic image. In the initial stage when the first tomographic image and the second tomographic image have not yet been obtained, the energy function setting unit 33 may set the energy function assuming that each of the first tomographic image and the second tomographic image is a certain initial image (for example, an image in which all pixels have a constant pixel value). Details of the processing of the first image update unit 31, the second image update unit 32, and the energy function setting unit 33 will be described using FIG. 5.

[0033] 5 is a flowchart of an image reconstruction method. This image reconstruction method includes steps S1, S2, an energy function setting step S3 performed by energy function setting unit 33, a first image updating step S4 performed by first image updating unit 31, a second image updating step S5 performed by second image updating unit 32, steps S6, and S7. The processing of each step of this image reconstruction method is performed according to a program stored in storage unit 34. The energy function setting step S3, the first image updating step S4, and the second image updating step S5 are repeated N times, where n=0 to n=N-1, where N is an integer greater than or equal to 2.

[0034] In the following, the first tomographic image is p The number of pixels in the first tomographic image is J. The initial image of the first tomographic image is expressed as (x p ) (0) and the pixel value of the j-th pixel of the initial image is expressed as (x p ) (0) j The first tomographic image generated by updating in the n-th (n is an integer of 0 to (N-1)) first image updating step S4 is expressed as (x p ) (n+1) and the pixel value of the j-th pixel of the first tomographic image is expressed as (x p) (n+1) j The first measurement data is expressed as y p and the number of elements of the first measurement data is I p The i-th component of the first measurement data is (y p ) i The system matrix describing the relationship between the first measurement data and the first tomographic image is expressed as a p The element in the i-th row and j-th column of the system matrix is ​​expressed as (a p ) ij It is expressed as:

[0035] Similarly, the second tomographic image is x c The number of pixels of the second tomographic image is J. The initial image of the second tomographic image is expressed as (x c ) (0) and the pixel value of the j-th pixel of the initial image is expressed as (x c ) (0) j The second tomographic image generated by updating in the n-th (n is an integer of 0 to (N-1)) second image updating step S5 is expressed as (x c ) (n+1) and the pixel value of the j-th pixel of the second tomographic image is expressed as (x c ) (n+1) j The second measurement data is expressed as y c and the number of elements of the second measurement data is expressed as I c The i-th component of the second measurement data is (y c ) i The system matrix describing the relationship between the second measurement data and the second tomographic image is expressed as a c The element in the i-th row and j-th column of the system matrix is ​​expressed as (a c ) ij It is expressed as:

[0036] In step S1, a first tomographic image x p The initial image (x p ) (0) and the second tomographic image x c The initial image (x c ) (0)These initial images may be, for example, images in which all pixels have a constant pixel value. In step S2, the value of n is set to 0.

[0037] In the energy function setting step S3, the energy function setting unit 33 sets the first tomographic image (x p ) (n) and the second tomographic image (x c ) (n) The energy function U that expresses the degree of difference between (n) The energy function U set when processing n=0 (0) is the initial image of the first tomographic image (x p ) (0) and the initial image of the second tomographic image (x c ) (0) An example of the energy function will be described later.

[0038] In the first image update step S4, the first image update unit 31 updates the first measurement data y p and the energy function U (n) Based on the update formula of the MAP reconstruction method, the first tomographic image (x p ) (n) (x p ) (n+1) The update formula in the n-th first image update step S4 is expressed by, for example, the following formula (1).

[0039]

number

[0040] In the second image update step S5, the second image update unit 32 updates the second measurement data y c and the energy function U (n) Based on the update formula of the MAP reconstruction method, the second tomographic image (x c ) (n) (x c ) (n+1) The update formula in the n-th second image update step S5 is expressed by, for example, the following formula (2).

[0041]

Number

[0042] In step S6, it is determined whether the value of n has reached N - 1. If n = N - 1, the process ends. If n < N - 1, after increasing the value of n by 1 in step S7, the process returns to step S3. As a result, the energy function setting step S3, the first image update step S4, and the second image update step S5 are repeated N times.

[0043] <00002�5>The MAP reconstruction method is based on a statistical method based on maximum a posteriori (MAP) estimation, and is obtained by adding prior information (prior knowledge) to successive approximation reconstruction methods such as the MLEM (Maximum Likelihood Expectation Maximization) method and the OSEM (Ordered Subsets Expectation Maximization) method. In the case of this embodiment, information that the first tomographic image and the second tomographic image should be equal to each other is used as prior information. The prior probability P representing this prior information is expressed by the following formula (3) using the energy function U. β is a parameter for controlling the sharpness of the prior probability, and Z is a parameter for making the total sum of probabilities equal to 1. The closer each of the first tomographic image and the second tomographic image is to the correct image, the smaller the value of the energy function U. The update formulas of the above formulas (1) and (2) are based on the OSL (one-step-late) method proposed by Green (Non-Patent Document 2). In addition, in this embodiment, an update formula proposed other than this may also be used.

[0044]

Number

[0045] The energy function U represents the degree of difference between the first and second tomographic images, and various functional formulas are possible. The greater the difference between the first and second tomographic images, the larger the function value of the energy function U. Preferably, the amount of change in the function value with respect to an increment of the difference is larger when the difference is between the first and second thresholds than when the difference is between 0 and the first threshold and when the difference is greater than the second threshold. The energy function U is preferably expressed by the functional formula (4) below, and is also preferably expressed by the functional formula (5) below. FIG. 6 is a diagram showing the functional formula (4). FIG. 7 is a diagram showing the functional formula (5). In these figures, the horizontal axis corresponds to the difference between the first and second tomographic images.

[0046]

number

[0047]

number

[0048] Next, the conditions and results of the simulation will be explained with reference to Figs. 8 to 12. The scattering detector 11 assumed in the simulation had a structure in which 30 x 30 x 1 scatterers made of silicon were stacked. The size of each scatterer was 1.0 x 1.0 x 6.0 mm. 3 The assumed absorption detector 21 had a structure in which scintillators consisting of 16 x 16 x 4 GSOZs were stacked. The size of each scintillator was 2.85 x 2.85 x 7.5 mm. 3 Instead of the subject 2, a case was assumed in which a phantom 3 shown in FIG. 8 was placed in the measurement space.

[0049] FIG. 8 shows the configuration of the phantom 3 assumed in the simulation. The assumed overall shape of the phantom 3 was a cylinder with a central axis in the body axis direction (Z-axis direction in the figure), with top and bottom diameters of 220 mm and a length in the body axis direction of 150 mm. The phantom 3 had multiple RI source regions (black-filled regions in the figure) 52 arranged within a background region (hatched region in the figure) 51. The amount of RI source in the background region 51 was 175 Bq / mL, and the amount of RI source in each RI source region 52 was 125 Bq. Each RI source region 52 was cylindrical with a diameter of 0.25 mm and a length of 0.25 mm. The RI source regions 52 were arranged on a certain XY plane, with one RI source region 52 located at the center of the plane. The RI source regions 52 were arranged at 30-mm intervals on each of five radial lines from the center position.

[0050] Under these conditions, simulations were performed for each of the comparative example, example 1, and example 2 to reconstruct the first and second tomographic images. In comparative example 1, the tomographic image was reconstructed using the OSEM method. In example 1, the function formula (4) was used as the energy function U, and β=1×10 -7 and δ = 1 × 10 4 In Example 2, the energy function U is expressed by the function formula (5), and β=1×10 -6 and δ = 2 × 10 4 Then, the tomographic images were reconstructed using the MAP reconstruction method.

[0051] 9 to 11 are diagrams showing tomographic images reconstructed by simulation. FIG. 9(a) shows a first tomographic image reconstructed in the comparative example. FIG. 9(b) shows a second tomographic image reconstructed in the comparative example. FIG. 10(a) shows a first tomographic image reconstructed in Example 1. FIG. 10(b) shows a second tomographic image reconstructed in Example 1. FIG. 11(a) shows a first tomographic image reconstructed in Example 2. FIG. 11(b) shows a second tomographic image reconstructed in Example 2. Comparing the first tomographic images (each figure (a)), which are PET images, Example 1 has lower noise than the comparative example, and Example 2 has even lower noise. Furthermore, comparing the second tomographic images (each figure (b)), which are Compton images, Example 1 has higher quality than the comparative example, and Example 2 has even higher quality.

[0052] FIG. 12 is a graph showing the relationship between the resolution and noise of the first tomographic image reconstructed by simulation. This graph shows the resolution and noise of the first tomographic image when the number of repetitions N is set to various values ​​for the comparative example, Example 1, and Example 2. The horizontal axis represents the noise (Coefficient of Variation, COV) in the background region 51 of the phantom 3. The vertical axis represents the average value of the full width at half maximum (FWHM) of each RI source region 52 of the phantom 3. The three points within the elliptical range in the graph represent the resolution and noise of the first tomographic image when the number of repetitions N is the same for the comparative example, Example 1, and Example 2. As can be seen from this graph, Example 1 is superior to the comparative example in both high resolution and low noise, and Example 2 is even superior.

[0053] As described above, according to this embodiment, it is possible to reconstruct a high-quality tomographic image with improved spatial resolution while suppressing statistical noise based on measurement data collected by a WGI device.

[0054] In the above description of the embodiment, the second image update unit 32 uses the second measurement data collected for a single counting event in which one of a pair of gamma rays generated by positrons emitted from a positron-emitting isotope in the subject 2 is Compton-scattered in one of the scattering detectors 11 and then photoelectrically absorbed in one of the absorption detectors 21 and detected. However, in the case of a positron-emitting isotope (for example 89 When a drug labeled with Zr) is administered to the subject 2, the second image update unit 32 may use second measurement data collected for a single counting event in which a single gamma ray emitted from a positron-emitting isotope in the subject 2 is Compton scattered in any one of the scattering detectors 11 and then photoelectrically absorbed and detected in any one of the absorption detectors 22. In this case, the same effect can be obtained by the same processing. [Explanation of symbols]

[0055] 1...radiation tomography system, 2...subject, 3...phantom, 10...scatter detector ring, 11...scatter detector, 20...absorption detector ring, 21...absorption detector, 30...image reconstruction device, 31...first image update unit, 32...second image update unit, 33...energy function setting unit, 34...memory unit, 40...recording medium, 51...background region, 52...RI source region.

Claims

1. 1. An apparatus for reconstructing a tomographic image of a subject based on measurement data collected by a scattering detector ring including a plurality of scattering detectors provided around a measurement space in which a subject, to which a drug labeled with a positron radioisotope is administered, is placed, and an absorption detector ring including a plurality of absorption detectors provided around the scattering detector ring, comprising: a first image updating unit that updates a first tomographic image of the subject; a second image updating unit that updates a second tomographic image of the subject; and an energy function setting unit that sets an energy function that represents a degree of difference between the first tomographic image and the second tomographic image, the first image update unit uses first measurement data collected about a coincidence event in which a pair of gamma rays generated by positrons emitted from a positron radioisotope in the subject are photoelectrically absorbed and detected in any two of the plurality of absorption detectors, and updates the first tomographic image by an update formula of a MAP reconstruction method based on the first measurement data and the energy function; the second image update unit uses second measurement data collected for a single counting event in which one gamma ray of a pair of gamma rays generated by a positron emitted from a positron radioisotope in the subject is Compton scattered in one of the plurality of scattering detectors and then photoelectrically absorbed in one of the plurality of absorption detectors, and updates the second tomographic image by an update equation of a MAP reconstruction method based on the second measurement data and the energy function; repeating the processes of the energy function setting unit, the first image updating unit, and the second image updating unit multiple times while updating the energy function set by the energy function setting unit; Image reconstruction device.

2. 1. An apparatus for reconstructing a tomographic image of a subject based on measurement data collected by a scattering detector ring including a plurality of scattering detectors provided around a measurement space in which a subject is placed, the subject having been administered a drug labeled with a positron-emitting isotope that also emits single gamma rays, and an absorption detector ring including a plurality of absorption detectors provided around the scattering detector ring, a first image updating unit that updates a first tomographic image of the subject; a second image updating unit that updates a second tomographic image of the subject; and an energy function setting unit that sets an energy function that represents a degree of difference between the first tomographic image and the second tomographic image, the first image update unit uses first measurement data collected about a coincidence event in which a pair of gamma rays generated by positrons emitted from a positron radioisotope in the subject are photoelectrically absorbed and detected in any two of the plurality of absorption detectors, and updates the first tomographic image by an update formula of a MAP reconstruction method based on the first measurement data and the energy function; the second image update unit uses second measurement data collected for a single counting event detected when a single gamma ray emitted from a positron radioisotope in the subject is Compton scattered in any one of the plurality of scattering detectors and then photoelectrically absorbed in any one of the plurality of absorption detectors, and updates the second tomographic image using an update equation for a MAP reconstruction method based on the second measurement data and the energy function; repeating the processes of the energy function setting unit, the first image updating unit, and the second image updating unit multiple times while updating the energy function set by the energy function setting unit; Image reconstruction device.

3. The energy function is the greater the difference between the first tomographic image and the second tomographic image, the greater the function value; a change in the function value relative to an increment of the difference is larger when the difference is in a range from the first threshold value to the second threshold value than when the difference is in a range from 0 to the first threshold value and a range greater than the second threshold value; 3. The image reconstruction device according to claim 1.

4. a scattering detector ring including a plurality of scattering detectors provided around a measurement space in which a subject administered with a drug labeled with a positron-emitting isotope is placed; an absorption detector ring including a plurality of absorption detectors disposed around the scattering detector ring; an image reconstruction device according to claim 1 or 2, which reconstructs a tomographic image of the subject based on measurement data collected by the scattering detector ring and the absorption detector ring; A radiation tomography system comprising:

5. A method for reconstructing a tomographic image of a subject, to which a drug labeled with a positron-emitting isotope has been administered, based on measurement data collected by a scattering detector ring including a plurality of scattering detectors provided around a measurement space in which the subject is placed, and an absorption detector ring including a plurality of absorption detectors provided around the scattering detector ring, comprising: a first image updating step of updating a first tomographic image of the subject; a second image updating step of updating a second tomographic image of the subject; and an energy function setting step of setting an energy function representing a degree of difference between the first tomographic image and the second tomographic image, the first image updating step uses first measurement data collected about a coincidence event in which a pair of gamma rays generated by positrons emitted from a positron radioisotope in the subject are photoelectrically absorbed and detected by any two of the plurality of absorption detectors, and updates the first tomographic image by an update formula of a MAP reconstruction method based on the first measurement data and the energy function; the second image updating step uses second measurement data collected for a single counting event detected when one of a pair of gamma rays generated by a positron emitted from a positron radioisotope in the subject is Compton scattered in one of the plurality of scattering detectors and then photoelectrically absorbed in one of the plurality of absorption detectors, and updates the second tomographic image using an update equation for a MAP reconstruction method based on the second measurement data and the energy function; repeating the processes of the energy function setting step, the first image updating step, and the second image updating step a plurality of times while updating the energy function set in the energy function setting step; Image reconstruction method.

6. A method for reconstructing a tomographic image of a subject based on measurement data collected by a scattering detector ring including a plurality of scattering detectors arranged around a measurement space in which a subject is placed, the subject having been administered a drug labeled with a positron-emitting isotope that also emits single gamma rays, and an absorption detector ring including a plurality of absorption detectors arranged around the scattering detector ring, comprising: a first image updating step of updating a first tomographic image of the subject; a second image updating step of updating a second tomographic image of the subject; and an energy function setting step of setting an energy function representing a degree of difference between the first tomographic image and the second tomographic image, the first image updating step uses first measurement data collected about a coincidence event in which a pair of gamma rays generated by positrons emitted from a positron radioisotope in the subject are photoelectrically absorbed and detected by any two of the plurality of absorption detectors, and updates the first tomographic image by an update formula of a MAP reconstruction method based on the first measurement data and the energy function; the second image updating step uses second measurement data collected for a single counting event detected when a single gamma ray emitted from a positron-emitting isotope in the subject is Compton-scattered in any one of the plurality of scattering detectors and then photoelectrically absorbed in any one of the plurality of absorption detectors, and updates the second tomographic image using an update formula for a MAP reconstruction method based on the second measurement data and the energy function; repeating the processes of the energy function setting step, the first image updating step, and the second image updating step a plurality of times while updating the energy function set in the energy function setting step; Image reconstruction method.

7. The energy function is the greater the difference between the first tomographic image and the second tomographic image, the greater the function value; a change in the function value relative to an increment of the difference is larger when the difference is in a range from the first threshold value to the second threshold value than when the difference is in a range from 0 to the first threshold value and a range greater than the second threshold value; 7. The image reconstruction method according to claim 5 or 6.

8. 7. A program for causing a computer to execute each step of the image reconstruction method according to claim 5 or 6.

9. A computer-readable recording medium on which the program according to claim 8 is recorded.

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